Improve js_os_exec (#295)

- use $(shell) make command to test if closefrom() is available
- use closefrom() if available in js_os_exec()
- limit the fallback loop to 1024 handles to avoid costly loop on linux alpine.
PR inspired by @nicolas-duteil-nova

Makefile

@@ -143,6 +143,11 @@ endif
 ifdef CONFIG_WIN32
 DEFINES+=-D__USE_MINGW_ANSI_STDIO # for standard snprintf behavior
 endif
+ifndef CONFIG_WIN32
+ifeq ($(shell $(CC) -o /dev/null compat/test-closefrom.c 2>/dev/null && echo 1),1)
+DEFINES+=-DHAVE_CLOSEFROM
+endif
+endif
 
 CFLAGS+=$(DEFINES)
 CFLAGS_DEBUG=$(CFLAGS) -O0

compat/test-closefrom.c

@@ -0,0 +1,6 @@
+#include <unistd.h>
+
+int main(void) {
+    closefrom(3);
+    return 0;
+}

quickjs-libc.c

@@ -3015,7 +3015,6 @@ static JSValue js_os_exec(JSContext *ctx, JSValueConst this_val,
     }
     if (pid == 0) {
         /* child */
-        int fd_max = sysconf(_SC_OPEN_MAX);
 
         /* remap the stdin/stdout/stderr handles if necessary */
         for(i = 0; i < 3; i++) {
@@ -3024,9 +3023,28 @@ static JSValue js_os_exec(JSContext *ctx, JSValueConst this_val,
                     _exit(127);
             }
         }
+#if defined(HAVE_CLOSEFROM)
+        /* closefrom() is available on many recent unix systems:
+           Linux with glibc 2.34+, Solaris 9+, FreeBSD 7.3+,
+           NetBSD 3.0+, OpenBSD 3.5+.
+           Linux with the musl libc and macOS don't have it.
+         */
 
+        closefrom(3);
+#else
+        {
+            /* Close the file handles manually, limit to 1024 to avoid
+               costly loop on linux Alpine where sysconf(_SC_OPEN_MAX)
+               returns a huge value 1048576.
+               Patch inspired by nicolas-duteil-nova. See also:
+               https://stackoverflow.com/questions/73229353/
+               https://stackoverflow.com/questions/899038/#918469
+             */
+            int fd_max = min_int(sysconf(_SC_OPEN_MAX), 1024);
             for(i = 3; i < fd_max; i++)
                 close(i);
+        }
+#endif
         if (cwd) {
             if (chdir(cwd) < 0)
                 _exit(127);